Broadcast television signals are usually provided in an interlaced form. For example, the phase alternate line (PAL) system used in Europe is made of video frames comprising two interlaced fields. Each field comprises alternate is lines of the frame. Thus when the signal is applied to a display, the first field will be applied to the odd numbered lines of the display followed by the second field being to the even numbered lines of the display. Frame rate, the rate at which frames comprising two interlaced fields are applied to a display is usually 50 Hz, and therefore field rate is 100 Hz. Thus, if each field is converted to a whole frame of video data, i.e. the missing lines in each field are somehow generated, the effective frame rate will be 100 Hz. This also has the advantage of increasing the resolution of the television picture.
There are various ways that have been proposed to generate the missing lines in a field of video data. Typically, the missing lines are generated by an interpolation technique using adjacent pixels (adjacent either temporarily or spatially, or both) to generate each missing pixel.
FIG. 1 illustrates schematically three different possible interpolation schemes using spatially adjacent pixels to generate the missing pixels. The middle scheme shown comprises correlation of the data from pixels above and below the pixel to be reconstructed and correlation of data from pairs of pixels immediately adjacent to this. The scheme on the left hand side of FIG. 1 comprises correlation between pixels on lines which pass diagonally sloping down to the right of the pixel being reconstructed. The same process with the opposite diagonals is shown in the right hand example of FIG. 1.
In the examples shown in FIG. 1, the correlation between the data and the various pairs of pixels are derived using the sum of absolute differences (SAD) technique.
A comparison is made between the various interpolation schemes, e.g. SAD0, SAD1, SAD2 etc as illustrated in FIG. 1, in order to select the best one. This is shown graphically on the right hand side of FIG. 1. This graph of the SAD results of the possible interpolation schemes is called the correlation curve. Typically, the interpolation scheme which gives the smallest difference in SAD is used as the interpolation to reconstruct the missing pixel. The process is carried out for each pixel, so that different interpolation schemes may be used for adjacent pixels.
An interpolation scheme of this type, using both spatial and temporal interpolation, is described in more detail in GB 2422974. This type of interpolation scheme generally gives goods results. However, there is a problem that such a scheme can yield a high frequencies within a reconstructed line of video data when the interpolation scheme used jumps rapidly from pixel to pixel. This problem is greatest when the minimum of the correlation curve jumps from one side of the curve to the other for adjacent pixels.
Accordingly there is a need to improve the interpolation techniques of the prior art and in particular to prevent the generation of high frequencies along the reconstructed scan line owning to rapid changes in selected interpolation schemes.